Dropbox uses a combination of wxPython and PyObjC on the Mac (less wxPython in the 0.8 series). It looks like they’ve built a bit of a UI abstraction layer but nothing overwhelming—i.e., they’re doing their cross-platform app the right way.
They include their own Python mainly because the versions of Python included on the Mac vary by OS version (and Dropbox supports back to 10.4 IIRC); also, they’ve customized the Python interpreter a bit to improve threading and I/O behavior.
(I do not work for Dropbox or have any inside knowledge; all I did was read their forums and examine the filenames in site-packages.zip in the Dropbox app bundle.)